The use of electronics in HVAC systems has become increasingly common in recent years and has grown to include heat pumps and electric furnaces. The use of electronics, with relays, to control electric heat, has only recently become practical through the use of zero, or near zero, voltage crossing switching techniques such as those disclosed and claimed in co-assigned U.S. Pat. No. 5,530,615, the contents of which is included herein by this reference. With respect to defrost controls, electronics have replaced electro-mechanical controls for a considerable period of time.
Conventional split systems for residential heat pumps have an indoor evaporator coil unit and an outdoor condenser coil unit with electronic controls for each unit receiving an input from a wall thermostat for either heating or cooling and for operating outputs such as electric resistive heat, fans, reversing valves and a compressor. Typically, some seven wires are required to interconnect the thermostat and the indoor control with six wires running out to the outdoor unit. It would be very desirable to reduce the wiring complexity from a standpoint of cost saving but also because many field failures occur due to miswiring during installation and decreasing the wiring connections would result in fewer failures. Even when the wiring is done correctly, however, there are undesirable functional limitations of the conventional control system. Ideally, the indoor fan should not be energized when the compressor is not operating, however, there are certain operational modes in which the indoor fan can be energized when the compressor is not energized such as in a lockout, either for a short or a long duration.